Domestic flight. The solution may be I can't take a privileged thing on the plane and maintain the privilege. I don't waive the privilege if I mail it, so it's still possible to transport privileged things, but in luggage aboard a commercial flight. I don't know if there's really an answer, and the "thing" issue is a pure hypothetical, something that rarely comes up in legal practice. I do fly regularly with privileged documents, however.

In chat I stuck with documents, as that's an easier and more real situation. But there can be attorney-client privileged things. Crazy hypothetical: Say my client somehow acquired a heavy gold necklace we know was stolen and gives it to me for safe-keeping and I need to travel with it. The necklace isn't dangerous, and so there's no reason for security to stop me from taking it on the plane, but the fact I have it, and my client ever had it, is privileged, but there it'll be when my luggage is X-rayed.